Solvothermal preparation of luminescent zinc(II) and cadmium(II) coordination complexes based on the new bi-functional building block and photo-luminescent sensing for Cu²⁺, Al³⁺ and L-lysine

Highlights

• Two new coordination complexes based on 4′-(1H-1,2,4-triazole-1-yl)-[1,1′-biphenyl]-4-carboxylic acid have been synthesized.

• Photo-luminescent experiment reveals that 1 can accurately recognize Al³⁺ and Cu²⁺with excellent selectivity and sensitivity.

• 1 can recognize L-lysine with excellent sensitivity (K_sv = 4.9118 × 10⁴ [M]⁻¹).

Abstract

In industry, over usage of Cu²⁺ and Al³⁺ will lead to toxic wastewater, which further to give serious pollution for the environment. On the other hand, L-lysine can enhance serotonin release in the amygdala, with subsequent changes in psychobehavioral responses to stress. Therefore it is the urgent problem to design a method for detecting the amount of Cu²⁺, Al³⁺, and L-lysine. In this work, through the solvothermal synthesis method, two new coordination complexes based on the new bifunctional building block 4′-(1H-1,2,4-triazole-1-yl)- [1,1′-biphenyl]-4-carboxylic acid (HL) have been synthesized, namely, [Zn(L)₂·4H₂O] (complex 1) and [Cd(L)₂·4H₂O] (complex 2). X-ray single-crystal diffractometer was used to analyze its structure, powder X-ray diffraction (PXRD) patterns confirmed that 1 and 2 powder’s purity and 1 can keep stable during the detection process of Cu²⁺, Al³⁺, and L-lysine, respectively. Elemental analysis, thermogravimetric analysis, infrared analysis, ultraviolet analysis and fluorescent spectrum have been used to characterize these complexes. The photo-luminescent test showed that 1 can accurately recognize Al³⁺ and Cu²⁺ among various cations. On the other hand, 1 can distinguish L-lysine among amino acid molecules. Therefore, 1 can be utilized as a multifunctional fluorescent probe for Al³⁺(K_sv = 1.5570 × 10⁴ [M]⁻¹), Cu²⁺(K_sv = 1.4948 × 10⁴ [M]⁻¹) and L-lysine (K_sv = 4.9118 × 10⁴ [M]⁻¹) with low detection limits (17.5 μM, 18.2 μM, 5.6 μM) respectively.

Introduction

In recent years, metal-organic coordination complexes have become a research hotspot for new materials [1], not only because of their novel structure and topology but also due to their potential application value in fluorescence [2], gas storage [3], magnetism [4], catalysis [5], etc. With the continuous development of coordination chemistry [6], especially the development of transition metal-based complexes [7], its application in many fields is a hot topic of research, such as pharmacology [8], life sciences, and industrial catalysis [9], coordination chemistry has become the frontier of chemistry research and has been greatly promoted [10]. Especially, the fluorescence properties of coordination complexes also attract many scientists’ attention due to fast response, high sensitivity, low cost, and easy operation [11].

Triazole ligands have aroused great interest of chemists and materials scientists due to their modifiable properties, flexible coordination modes, and the diverse structures and excellent properties of their complexes. In terms of fluorescent materials, 1,2,4,-triazole compounds have an electron-rich π-conjugated system, and contain sp² hybridized imidazole nitrogen atoms and sp³ hybridized amino nitrogen atoms, which are an excellent luminescence performance of the chromophore group [12]. It has potential application value in the field of fluorescent materials research. Carboxylic acid-based ligand is a good candidate for a rod in the assembly with the high coordinated ability for the lanthanide metal salts, which could induce the construction of MOFs with novel topologies, such as the unusual polyrotaxane-like MOFs, interlocked and interdigitated architectures, and intriguing three-dimensional (3D) polycatenated arrays featuring an uneven “density of catenation” [13], [14], [15], [16]. However, there are few studies on the optical properties of complexes based on organic ligands simultaneously containing 1,2,4-triazole and carboxylic groups. Therefore, studying the fluorescence properties of transition metal complexes based on the bifunctional ligand is a frontier research field, and this research direction has left a broad space for scientists to explore.

The fluorescent probe has the advantages of strong specificity, high sensitivity, convenient and quick operation, and short response time. It has shown great potential for application in analytical chemistry, biological sciences, clinical biochemistry, and environmental sciences. To obtain high-selectivity and high-sensitivity cation fluorescent probes, great efforts have been put into this field. Some examples of complexes used in fluorescence sensing have been reported [17]. In Shankar’s work [18], they isolated binuclear Zn(II) and Cu(II) complexes based on a new Schiff base ligand N,N'-bis(2-hydroxy benzylidene)-2,4,6-trimethyl benzene-1,3-diamine namely [Zn₂L₂] (1) and [Cu₂L₂]·H₂O (2). Selective On-Off-On switching behavior of the fluorescent complex 1 was studied. The fluorescence intensity of 1 can quench in the turns-off mode upon addition of Cu²⁺, while enhances in the turns-on mode in the presence of Ag⁺ ions. The mechanisms of On-Off-On signaling were supported by NMR, ESI-MS, electronic absorption, and emission spectral studies. Job's plot supported 1:1 and 1:2 stoichiometries for Cu²⁺ and Ag⁺ ions. Association and quenching constants were estimated by the Benassi-Hildebrand method and Stern-Volmer plot. In Wang’s work, through the hydrothermal method a three-dimensional (3D) cluster-based metal-organic frameworks (MOFs) have been synthesized, namely, {[Gd(L)(H₂O)(DMF)]·DMF}_n. This material can be used to detect Al³⁺ with high sensitivity in human serum solution [19]. According to the mentioned above, we can found that it is meaningful to design a detection system to control the concentration of Al³⁺ and Cu²⁺ which to control the amount and further to avoid water pollution to the environment and decreasing the risk of diabetes.

Copper is the abundant transition metal in the human body after iron and zinc, which plays an important role in various physiological and pathological processes. As an important catalytic cofactor for many enzymatic reactions, copper ion can be involved in the cellular metabolism of all organisms. Perturbed copper homeostasis also easily leads to numerous diseases, especially those related to brain function activity, including Alzheimer’s disease. Excessive copper has the cytotoxicity, which can lead to damage to human health, it also the main factor in the industrial wastewater. Recently, Gasser’s group synthesized the Tb(III) complex and make it a suitable candidate for implementation into a sensor of Cu²⁺ for aqueous conditions [20]. Besides, aluminum is extensively involved in many civil and industrial processes. Especially, Al³⁺ ion existing in water and plants may be harmful to the human body through foods and drinking water. The excessive accumulation of aluminum in the body will lead to illnesses such as Alzheimer’s disease, Parkinsonism dementia. Recently, Zhai and his co-workers designed the excited-state intramolecular proton transfer (ESIPT)-based fluorescent MOF sensor, which is used for quantitative Al³⁺ detection, together with the extraordinary sensitivity, selectivity, fast response, and good reusability [21].

Amino acids are the “basic units” that make up various proteins in the human body. Amino acid detection has an important prompting effect on the metabolic status and disease conditions in some patients [22]. Serine helps the production of immunoglobulin and antibodies and plays an active role in immune protection. Arginine has an important metabolic function is to promote wound healing[23]. Arginine is also a component of human urea metabolism [24]. If the patient lacks arginine, it will lead to hyper ammonia and even coma. Lysine is an essential amino acid for the human body. It is essential for the production of white blood cells, the synthesis of pepsin, the secretion of gastric acid, and the balance of potassium and sodium ions. For the detection of amino acids, some scientists have made some explorations. In Wang’s work, a 2D water-stable silver(I) cation metal–organic frameworks, namely 2D MOF {[Ag(L)₂]BF₄}_n, this material can distinguish L- and D-Cysteine from other amino acids in real-time through the fluorescence quenching effect [25]. As for the L-lysine, little is known about the psychobehavioral consequences of a dietary deficiency of the amino acid, L-lysine. According to the Kunio group’s work [26], they found that a 4-d long L-lysine deficiency in rats interfered with the normal circadian release of the neurotransmitter serotonin, but not dopamine, measured by in vivo microdialysis in the central nucleus of the amygdala. They concluded that a severe deficiency of dietary L-lysine enhances serotonin release in the amygdala, with subsequent changes in psychobehavioral responses to stress. So it is meaningful that design a useful detection system for L-lysine to control its concentration in the human body.

In this work, we synthesized two complexes based on transition metals Zn and Cd by the solvo-thermal synthetic method. Complexes 1 and 2 have been characterized by single-crystal X-ray diffraction, powder X-ray diffraction (PXRD), Fourier Transform Infrared Spectroscopy, UV–vis absorption spectrum, TG analysis and scanning electron microscopy (SEM). PXRD patterns of the as-synthesized samples 1 and 2 have confirmed the purity of the bulky samples. And the experimental results show that complex 1 can distinguish Cu²⁺ and Al³⁺ from other cations with excellent selectivity and sensitivity. Additionally complex 1 can also selectively recognize L-lysine from a large number of amino acid molecules with a low detection limit (5.6 μM).